A brochure from the company Endress+Hauser "Pressure Transmitter cerabar PMC 133", Maulburg, Germany, TI 105P/00/e/02.94 describes a pressure measuring arrangement comprising:
a capacitive, ceramic pressure sensor, which has a substrate having at least one electrode applied thereto and an associated diaphragm having a counter-electrode applied thereto, PA1 a metallic housing having a wall, a removable cover, an intermediate bottom and an opening situated opposite to the latter, PA1 a current output circuit arranged inside the housing, between the intermediate bottom and the pressure sensor, PA1 a cable bushing for a cable leading to the pair of terminals. PA1 a capacitive, ceramic pressure sensor, which has a substrate having at least one electrode applied thereto and an associated diaphragm having a counterelectrode applied thereto, PA1 a pot-shaped metallic housing having a wall, a bottom and an opening situated opposite to the latter, PA1 a current output circuit arranged inside the housing, PA1 a shielding electrode which is arranged inside the housing in an insulated manner with respect to the wall and PA1 a potting compound, with which the interior of the housing is completely filled.
it being possible to make electrical contact with said electrode and said counter-electrode at a rear side of the substrate, PA2 into which opening the pressure sensor is inserted in a pressure-tight manner, PA2 which current output circuit converts an output signal of the converter circuit into a direct current which is proportional to the capacitance change, flows in two output lines leading to a pair of terminals arranged in the intermediate bottom and has a current intensity of between 4 mA and 20 mA, and PA2 it being possible to make electrical contact with said electrode and said counter-electrode at a rear side of the substrate, PA2 into which opening the pressure sensor is inserted in a pressure-tight manner, PA2 which current output circuit converts an output signal of the converter circuit into a direct current which is proportional to the capacitance change and flows in two output lines which pass in an insulated manner through the bottom of the housing, PA2 is coupled at low impedance to one of the two output lines and PA2 to the counter-electrode, and
on which rear side is arranged a converter circuit for capacitance changes between the electrode and counter-electrode which are caused by pressure changes, PA3 on which rear side is arranged a converter circuit for capacitance changes between the electrode and counter-electrode which are caused by pressure changes,
With respect to this previously described pressure measuring arrangement, one object of the invention is to further improve the protection against the coupling-in of interference signals, in other words the so-called electromagnetic compatibility, and furthermore to facilitate compliance with standards which ensure explosion protection.
In order to achieve this object, the invention consists in a pressure measuring arrangement comprising:
According to a preferred embodiment of the invention, a metal-coated plastic film serves as shielding electrode. In another preferred embodiment, the current output circuit generates a direct current having a current intensity of between 4 mA and 20 mA.
In a further preferred embodiment, the one output line and the counter-electrode are each electrically conductively connected to the shielding electrode.
In a further preferred embodiment, the one output line is electrically conductively connected to the shielding electrode and the counter-electrode is capacitively connected to the shielding electrode via a capacitor which is dimensioned in such a way that its capacitance represents the best possible short circuit for frequencies occurring in interference signals.
In a further preferred embodiment, the counter-electrode is electrically conductively connected to the shielding electrode and the one output line is capacitively connected to the shielding electrode via a capacitor which is dimensioned in such a way that its capacitance represents the best possible short circuit for frequencies occurring in interference signals.
Finally, in a further embodiment, the one output line and the counter-electrode are connected to the shielding electrode via a respective capacitor which is dimensioned in such a way that its respective capacitance represents the best possible short circuit for frequencies occurring in interference signals.
A first advantage of the invention consists in the fact that interference signals which are coupled in during the operation of the pressure measuring arrangement via two external connection lines, which are respectively connected to an output line, of a cable, in other words cable-conducted interference, are diverted to the housing by means of the low-impedance coupling, in particular electrically conductive or capacitive connection, of the one output line to the shielding electrode via the capacitor which is formed by the latter and the wall of the housing and has the potting compound as a dielectric, without it being possible for interference fields belonging to the interference signals to penetrate into the interior of the housing from the bottom.
A second advantage of the invention consists in the fact that interference signals which are captured by the counter-electrode are likewise diverted to the wall of the housing by means of the low-impedance coupling, in particular electrically conductive or capacitive connection, between the counter-electrode on the diaphragm of the pressure sensor and the shielding electrode, and, consequently, it is also not possible for any interference fields to penetrate into the interior of the housing from the opening in the latter.
A further advantage of the invention is to be seen in the fact that the potting compound makes it possible to comply with minimum lengths, prescribed in explosion protection standards, of creepage paths between parts which are at different potentials in the operating state. This relates particularly to those points where there is only a small distance between the counter-electrode of the diaphragm and the housing.